1. Field of the Invention
The present invention relates to a control apparatus for a power converter for controlling the output voltage of the power converter which has a plurality of switching elements in order to obtain DC converted output highly accurately.
2. Description of Related Art
For example, FIG. 9 shows the block connection diagram of a conventional converter control circuit illustrated in High Performance and Long Life Uninterruptable Power Source Using Flywheel Energy Storage Unit, pp. 1049-1055 in a bulletin of IEEE Industry Applications Society Annual Meeting (Oct. 7-12, 1990, Seattle).
Referring to FIG. 9, reference numeral 1 designates a load, numeral 2 designates a capacitor which smooths a DC voltage to be supplied to the load 1, and numeral 3 designates a converter main circuit. The converter main circuit has a single-phase full-bridge structure equipped with switching elements S1-S4, and voltage control diodes D1-D4 as shown in FIG. 10a, in order to perform pulse-width modulation using triangular-wave carrier of 1-2 KHz or more. Numerals 4 and 5 designate a reactor for a filter and a capacitor, respectively, which are provided between an AC power supply 6 and the aforementioned converter 3. Numeral 901 designates a current detecting device which detects converter current I.sub.A and numeral 902 designates a voltage detecting device which detects input voltage V.sub.R.
In FIG. 9, the 800 series designate the components of the control apparatus of the aforementioned converter 3. Numeral 801 designates a pulse-width modulating circuit (hereinafter referred to as PWM modulating circuit) which transmits control output PWMO for pulse-width-modulation for controlling the aforementioned converter 3 according to the converter voltage instruction V.sub.A *. Numeral 802 designates a current control amplifier which obtains the converter voltage instruction V.sub.A * to be supplied to the aforementioned PWM modulating circuit 801 according to the output of a subtractor 821 which will be described later. Numeral 803 designates a phase synchronizing circuit for obtaining the phase .phi. of the AC input V.sub.R according to a voltage detected by a voltage detecting device 902.
Numeral 804 designates an AC sine wave reference oscillating circuit for obtaining an AC sine wave reference signal sin .phi. having the same phase as the input voltage V.sub.R of the AC power supply. Numeral 805 designates an amplitude instruction generating circuit for sending an amplitude instruction. Numeral 821 is a subtractor for obtaining differences between a converter current instruction I.sub.A * obtained by a multiplier 822 which multiplies the AC sine wave reference signal sin .phi. sent from the aforementioned AC sine wave reference oscillating circuit 804 with an amplitude instruction sent from the aforementioned amplitude instruction generating circuit 805 and the converter current I.sub.A detected by the current detecting device 901 in order to transmit a signal representing such difference to the aforementioned current control amplifier 802. These components construct a current control loop.
The operation of the construction described above will be described below.
The input voltage V.sub.R from the AC power supply 6 is detected by means of the voltage detecting device and the phase .phi. of the input voltage V.sub.R is detected by means of the phase synchronizing circuit 803. The detected phase is input to the AC sine wave reference oscillating circuit 804 in order to obtain an AC sine wave reference signal sin .phi. having the same phase as the input voltage V.sub.R. The multiplier 822 multiplies the output of the amplitude instruction generating circuit 805 and the output of the aforementioned AC sine wave reference oscillating circuit 804 in order to obtain the converter current instruction I.sub.A *.
The current control amplifier 802 and the PWM modulating circuit 801 control switching action of the converter 3 to make the converter current instruction I.sub.A * agree with the converter current I.sub.A detected by the current detecting device 901, thereby forming a current control loop. Thus, under normal conditions, power can be supplied to the load 1 from the AC power supply 6 in the form of a sine wave current having a power factor of 1.
The control apparatuses of power converters of the prior art have the construction described above. For this reason, if there is even a slight distortion in input voltage, resonance is produced by the reactance component of this system and the filter capacitor 5. This is a problem of the prior art which must be solved. Further, if the control apparatus is stopped when power is being supplied to the load 1, energy stored in the reactance component of the system or energy stored in the reactor for filter 4 flow into the filter capacitor 5 causing the capacitor 5 be charged with excess voltage. This is another problem to be solved.